1. Technical Field
The present invention pertains to miniaturized chemical delivery systems. In particular, the present invention pertains to micro electromechanical systems for delivery of high purity chemicals to a manufacturing or other process.
2. Discussion of the Related Art
Micro electromechanical systems, or MEMS, are miniature systems used in a variety of applications. In particular, fluidic MEMS are used to transport and dispense gaseous and/or liquid fluid media for a number of applications. Fluidic MEMS include a variety of different components, including miniaturized valves, nozzles, pumps, orifices, sensors (e.g., pressure and/or temperature), reservoirs, etc. that include micro-channels sized on the order of microns or even nanometers. These MEMS components are typically manufactured utilizing one or a combination of conventional manufacturing techniques, such as bulk or surface micro-machining, high-aspect ratio micro-machining (HARM), and LIGA (referring to the German process of Lithographie/Galvanoformung/Abformung, or lithography/electroplating/molding).
The most advanced implementations of fluidic MEMS to date are in inkjet printer heads and micro-sensors utilized for biological and chemical analysis systems. The application of fluidic MEMS is highly desirable in other fluid distribution systems, such as high purity gas and liquid delivery and distribution systems for semiconductor fabrication and processing applications.
A number of different high purity liquid chemicals, including a wide spectrum of aqueous and organic solutions, are utilized for various production processes, such as chemical precursors for growing thin films in semiconductor and optics manufacturing (e.g., metal-organic compounds in solvents such as isopropanol, octane, tetrahydrofuran, toluene, etc.), as well as reactants, carriers and/or analytes in chemical, biochemical and pharmaceutical synthesis and analysis. Many of these chemicals can be easily contaminated (e.g., by exposure to air, particulate material, moisture, etc.) in the system distribution lines, resulting in reduced production yields or the formation of solids that may become embedded within and even clog distribution lines. In semiconductor manufacturing, for example, many metal-organic precursor chemicals react with oxygen and moisture, producing solid deposits that contaminate the distribution lines. In addition, many of the chemicals utilized in the distribution lines are toxic and/or present a fire hazard or explosion risk. Therefore, during long inactive periods when supply containers or sections of the distribution lines are disconnected from other parts of the system, the distribution lines often need to be subjected to an inert gas, a vacuum, and/or a liquid solvent to purge the lines of residual chemicals disposed therein, and to prevent the escape of chemicals into the surrounding atmosphere and also the interior of the distribution lines from intrusion of ambient air or other contaminants.
The delivery of chemicals within the high purity delivery system can be accomplished utilizing pressurization, pumping, and/or gravity. An inert gas, such as helium, is utilized to pressurize the chemicals in the distribution lines as well as chemical storage sources for supplying chemicals to the distribution lines. In order to provide a continuous delivery of one or more chemicals through the delivery system, often two or more containers are provided in series or in parallel with the distribution lines. When one container is empty, another container is brought on-line to continue supply of chemicals while the empty container is refilled.
While it is highly desirable to provide fluidic MEMS for high purity chemical distributions systems such as those noted above, the readily available and most commonly used equipment utilized for such distribution systems are conventional high purity valve and pipe fitting products, such as valves, VCRs, compression, flared, and pipe thread products commercially available from Swagelok (Solon, Ohio, e.g., NUPRO® products) and MKS Instruments, Inc. (Andover, Mass.). The conventional high purity fluidic components are adequate for many applications when a moderate number of components are necessary and higher chemical flows and/or pressures are required. However, such conventional components are less desirable when system size becomes an issue, particularly, when dealing with very low volumetric flows, e.g., 20 cubic centimeters per minute (ccm) or less. As the number of components (e.g., valves, sensors, mass flow controllers, etc.) increases, integration of conventional connectors and tubing into complex chemical distribution systems, not to mention conventional electronics control blocks (typically controlled by conventional programmable logic controllers), becomes cumbersome and quite expensive as well as requiring additional floor space to house the system. Further, these distribution systems are difficult to operate and control, as most individual components require dedicated control lines (electrical or pneumatic) and are more prone to contamination due to the increased system volume and the increasing difficulties associated with cleaning, purging and evacuating distribution lines within the system.
The implementation of MEMS into high purity systems would eliminate many of the problems associated with the bulkiness, contamination problems, increased expense and excessive size of complex fluid delivery systems utilizing conventional non-MEMS components. However, despite the advantages associated with employing MEMS, there has been a reluctance to utilize MEMS in high purity chemical distribution systems due to certain problems associated with MEMS components. In particular, MEMS shut-off valves, an important component in any fluid distribution system design, presently do not have sufficient sealing characteristics that would render such valves useful for complex distribution systems.
One of the most reliable and highly developed MEMS shut-off valves that presently exists is a cantilever-type valve, an example of which is the MEMS-Flow™ Ultra-Clean Shut-Off Valve commercially available from Redwood Microsystems, Inc. (Menlo Park, Calif.). A detailed disclosure of the MEMS-Flow™ valve of Redwood Microsystems, Inc. is described in U.S. Pat. No. 5,865,417, the disclosure of which is incorporated herein by reference in its entirety. In particular, this cantilever-type valve is formed by a micromachining process on a die or block and includes a cantilevered valve element and input and output channels disposed on in the block. In a normally closed position, the cantilevered valve element blocks an exit port within the valve to prevent fluid communication in a direction from the input channel to the output channel of the valve. The valve element is moved by a fluid filled membrane that flexes in response to energy inputs that heat the fluid within the membrane, forcing the valve element away from its sealing engagement so as to unblock the exit port and permit fluid to flow through the valve.
The problem associated with the cantilever-type design for MEMS shut-off valves is that they possess an asymmetrical sealing characteristic. In particular, these types of shut-off valves are “directional” and have a different leakage rate depending upon the direction of the pressure differential across the valve. In a normally closed position (i.e., the cantilever valve element blocks the exit port), leakage across the valve seal differs for the same pressure differential applied in the forward flow (i.e., inlet channel to outlet channel) orientation of the valve vs. the reverse flow (i.e., outlet channel to inlet channel) orientation.
In contrast, conventional shut-off valves, such as the Swagelok series of diaphragm shut-off valves, are less susceptible to valve leakage due to the development of positive and negative pressure differentials during system operation. Typical sealing requirements for shut-off valves used in semiconductor electronics manufacturing equipment is a leakage rate of no greater than about 1×10−9 Atm (measured with He)×standard cubic centimeters per second (i.e., Atm(He)*scc/sec). The cantilever-type shut-off valve design can yield a much larger leakage rate, particularly when negative pressure differentials develop across the valve approaching 20 psi (1.36 Atm) and greater.
Further, the integration of too many MEMS components into a single die may result in a reduced reliability and flexibility of the distribution system, where undesirable interactions or interference may occur between different sensors or actuators associated with the die.
In addition, the scaling of common types of fluid controllers into MEMS components can present problems. For example, the use of conventional MEMS mass flow controllers often becomes unreliable and presents quality issues, particularly in high purity chemical distribution applications, due to partial or even complete clogging of micro-channels or orifices that can occur during use of the mass flow controller. Frequently, such clogging and contamination problems occur when operation of the mass flow controller is halted for a period of time, resulting in residual and stagnant chemical fluid within the system block. Due to the extremely small internal dimensions and internal volume of the MEMS micro-channels, clogging of these channels with stagnant chemicals (e.g., due to particulates in the chemicals, precipitation of the chemicals, etc.) can occur with only a short interruption in flow through the flow controller block. The clogging problem can be minimized by scaling up the dimensions of the MEMS components. However, enlarging the size of the MEMS system diminishes the advantages associated with utilizing MEMS technology.
Thus, the integration of MEMS valves and other MEMS components into complex and miniature high purity fluid distribution systems is difficult and is not as simple as combining conventional, non-MEMS components together to form a particular chemical flow configuration.